<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    let arr = [1, 2, 3];
    Array.prototype.sum = function () {
      let total = 0;
      arr.map(item => {
        return total = total + item;
      })
      return total
    }
    arr.sum()
  </script>
</body>

</html>